The invention relates to electrical delay line circuits, and particularly to such circuits having an electrically variable delay with low jitter and stable insertion delay.
A "delay line" is a circuit device utilized to lower the velocity of propagation of an electrical signal so that the time required for the signal to travel the length of the delay line is increased with respect to the signal traveling along a conventional signal transmission line. Electrical delay lines are generally classified into two types: distributed-parameter devices and lumped-parameter devices. "A lumped-parameter" delay line is constructed with a number of sections by connecting a set of inductances in series, and a capacitance connected from each junction to a common junction that corresponds with a second conductor in a conventional two-wire transmission line, each section producing a small amount of delay. Lumped-parameter delay lines have heretofore been constructed having only fixed delays. Although manually adjustable, variable capacitors have been provided for adjusting the impedance of each section in a lumped-parameter delay line, such adjustable elements are utilized only for minimizing signal reflection, and are not electrically variable, i.e., variable by an electrical control signal.
It is desirable to construct a delay line having a repeatable, electrically variable delay for use in generating delays in the range of nanoseconds. Variable delays in the nanosecond range have heretofore been generated with complex circuit devices by applying the sloping level of a fast ramp signal to one input of a comparator, for comparison with a staircase signal applied to another input thereof, variations of delay being determined by the level of the staircase signal. Several factors degrade repeatability and contribute to instability of the resultant delay. Drift, in the range of microvolts, in the level of the staircase signal, and offset of the comparator circuit affect the stability of insertion delay. Jitter, which is determined by noise at the input of the comparator, is appreciable. Further, delay stability decreases as component count increases, and the implementation of fast-ramp comparators requires a large number of component elements in a plurality of circuits, viz.: staircase and fastramp signal generator circuits, a comparator circuit, and timing-signal generator circuits.
In view of the foregoing, it is an object of the present invention to provide improved means for delaying an electrical signal.
Another object of the present invention is to provide an improved electrically tunable delay line.
It is another object of the present invention to provide a tunable delay line generating delays in the nanosecond range which are repeatable and stable.